1. Field of the Invention
The present invention relates generally to implantable devices, such as synthetic vascular grafts and covered stents. More particularly, the present invention is directed to a method of impregnating an implantable device with substances as well as to an implantable device that has been so impregnated.
2. Description of the Related Art
Occlusion of blood vessels reduces or blocks blood flow. During the course of atherosclerosis, for example, growths called plaques develop on the inner walls of the arteries and narrow the bore of the vessels. An embolus, or a moving clot, is more likely to become trapped in a vessel that has been narrowed by plaques. Further, plaques are common sites of thrombus fonnation. Together, these events increase the risk of heart attacks and strokes.
Traditionally, critically stenosed atherosclerotic vessels have been treated with bypass surgery in which veins removed from the legs, or small arteries removed from the thoracic cavity, are implanted in the affected area to provide alternate routes of blood circulation. More recently, implantable devices, such as synthetic vascular grafts and stents, have been used to treat diseased blood vessels.
Synthetic vascular grafts are macro-porous vessel-like configurations typically made of expanded polytetrafluoroethylene (ePTFE), polyethylene terephthalate (PET), polyurethane (PU), or an absorbable polymer. Grafts made of ePTFE or PET are very non-wetting materials when introduced into an aqueous environment, causing difficulty in impregnating the materials. In addition, grafts made of ePTFE or PET are permanently implanted in the body, while grafts made of an absorbable polymer bioabsorb over time. A graft may be positioned into the host blood vessel as a replacement for a diseased or occluded segment that has been removed. Alternatively, a graft may be sutured to the host vessel at each end so as to form a bypass conduit around a diseased or occluded segment of the host vessel.
Intravascular stents may be permanently or temporarily implanted in narrowing blood vessels. Typically, a vascular catheter, or similar transluminal device, is utilized to carry the stent to the treatment site. The stent is expanded to support, and if desired expand, the diseased portion of the arterial wall. The insertion of a stent into a vessel in which plaque has become brittle over time, however, may cause complications if the stent causes the brittle plaque to break off from the vessel wall and embolize the lumen. A covered stent, in which a graft-like covering is slip-fit onto the stent, may therefore be employed to isolate the brittle plaque from direct contact with the stent, which is rigid.
To treat such damaged vasculature tissue and to help prevent thrombosis and restenosis, there is a need for administering therapeutic substances to the treatment site. To provide an efficacious concentration to the treatment site, systemic administration of the therapeutic substance often produces adverse or toxic side effects for the patient. Local delivery is a highly suitable method of treatment, in that smaller levels of therapeutic substances, as compared to systemic dosages, are concentrated at a specific site. Local delivery produces fewer side effects and achieves more effective results.
One technique for the local delivery of therapeutic substances is through the use of medicated coatings on implantable devices. In addition to providing local delivery capabilities, such a coating protects vessel tissue from injury and helps prevent excessive foreign body reaction that may occur when such a device is implanted.